Scanners typically use a CCD to transform an image into digital information. CCD's typically have sensitivity variations between the different cells or pixels in the CCD array. These non-uniformity's are typically corrected by doing a calibration scan before each user scan. Calibration scans typically correct for two types of non-uniformity, dark current and photo response non-uniformity. Dark current is the leakage current occurring in the CCD even when no light is hitting the CCD. Photo response non-uniformity is the variation in sensitivity between pixels to a constant amount of light hitting the CCD. Typically these problems are corrected by doing two scans, one with the light off, and one with the light on using a reference target of a known brightness. A calibration scan is described in U.S. Pat. No. 5,285,293 “Method and apparatus for providing sensor compensation” which is hereby incorporated by reference.
Currently these calibration scans take time, typically between 1 to 5 seconds. The total time is dependent on a number of factors. Some of the factors are: if the scanner light is turned off for dark noise calibration, how far the scan head must move to reach the calibration target, and how long the lamp takes to reach scanning intensity. In the past the calibration time was a small percentage of the total scan time. However with the increased speed of input/output channels, computers, and scanners, calibration scan times are becoming a larger percentage of the total scan time.
Today, some scanners turn off the scanner light at the end of each scan and other scanners leave the scan light on for extended periods of time. The trade off is the amount of energy used vs. the time it takes to turn on the light and reach a stable scanning intensity. The scanners that leave their lights on typically start scanning quicker than scanners that have to turn on their light first before they can scan. Typically the scanners that leave their light on use the same stored dark current calibration for each scan.
Currently scanners typically either do a calibration scan before each user scan or do one calibration scan at power-up and reuse this saved calibration scan. The scanners that do a calibration scan before each user scan, typically take longer to start the user scan than the scanners that just do one calibration scan at power-up. However, the saved calibration scan may not be as accurate as the calibration scan done before each user scan. This is because the lamp intensity profile can change over time and the temperature of the CCD can change over time. The calibration scan typically is used to correct lamp profiles as well as photo response non-uniformity and changes in the temperature of the CCD affect the amount of dark current.
Therefore there is a need for a calibration method that is accurate and still allows quick scan starts.